In the graphic arts, scribing is an improved drafting technique in which specially coated films are scribed by using sharp cutting tools to form desired lines, letters, numbers, and other symbols, in a thin surface coating on the drafting film. For example, a drafting film known as Scribe Coat.TM. manufactured by Keuffel & Esser Co., comprises a dimensionally stable polyester plastic film, approximately 0.0050 inch to 0.0075 inch in thickness, which is roller coated with actinically opaque paint. Drafting on this material is accomplished with various tools equipped with either phonograph needles, sapphire cutters or cutting blades. By removing the painted surface with these tools to form lines, letters, numbers and other symbols, a mechanical photographic negative is produced. This type of negative can be used directly to reproduce maps and other technical drawings by photolithographic techniques.
To form dots on such scribe films, drill-like drafting instruments commonly known as "dotters" have been developed in the prior art. Such instruments generally include a small electric motor coupled by a pulley drive arrangement to a rotatable chuck for holding a rotary cutting element. Typically, a manually operable control switch or toggle switch is provided to actuate the electric motor and rotate the cutting element. The cutting element is mounted for vertical movement along its axis. A push button operable by finger pressure is provided to lower the rotating cutter into contact with the scribe film to form a dot in its surface coating. After the dot is formed, the finger pressure is released to allow the cutting element to be raised out of contact with the scribe film.
In the operation of the prior art devices, once the control switch is turned on, the drive motor operates continuously regardless of whether the push button is depressed to lower the cutting element and scribe dots in the drafting film. Under this continuous operation, the electric drive motors tend to overheat and experience undue wear. As a result, such prior art devices are characterized by significant disadvantages: (1) the electric motors tend to burn out early and unnecessarily shorten the life of the units; (2) the units tend to waste electricity; (3) the units are expensive to operate (4) the electric motors tend to overheat causing stoppage and delays. In addition, the continuously rotating cutter element presents a safety hazard and can result in a serious finger injury.
In addition, the prior art devices typically make no provision for limiting the depth of penetration of the cutting element into the scribe film. Thus, even with the most careful attention to the scribing operation, it is easily possible to penetrate too far into the underlying plastic film and cause irreparable damage. Thus, it is extremely desirable to provide a scribing instrument designed to penetrate only the surface coating on the drafting film.
Accordingly, it is a primary object of the present invention to provide a drafting instrument for scribing dots in a drafting medium which is efficient and safe to operate.
Another object of the invention is to provide an improved drafting instrument wherein the drive motor is only actuated when it is desired to scribe dots in the drafting film.
It is also an object of the invention to provide an improved drafting instrument wherein the drive motor and cutter are movable as a single unit and the drive motor is automatically actuated upon downward movement by a predetermined distance before the cutter is moved into engagement with the drafting film.
A further object of the invention is to provide an improved drafting instrument wherein the depth of penetration of the cutter into the drafting film is limited to prevent damage to the drafting film.
It is another object of the invention to provide an improved drafting instrument wherein the drive motor is automatically turned off after the scribing is completed to enhance the safety of its operation.